Glass fibers are the oldest type of strong fibers used in applications such as composite structural materials. Although the possibility of forming fibers from heat-softened glass was known thousands of years ago, these fibers were discontinuous, and it was not until the 1930's that the production of continuous glass fiber became commercially viable. The first use for substantial quantities of continuous glass fiber was for electrical insulation of fine wires used at elevated temperatures. The continuous glass fibers for this application became known as "E" glass because of their electrical properties. "E" glass does not have a specifically defined composition, but is a type of glass of defined electrical properties, and is generally a low alkali, calcium aluminum borosilicate. "E" glass has an upper temperature use limit of about 1100.degree. F.
Improvements over the properties of "E" glass have been made through elimination of the alkali and other low melting components. This resulted in the development of "S" glass, known for its strength properties. "S" glass is a magnesium-aluminosilicate composition with considerably higher strength and modulus, and an upper temperature use limit of about 1500.degree. F.
The preparation of glass fiber with higher temperature use limits is desirable, and has been pursued through post treatment of "S" glass. U.S. Pat. No. 3,402,055 describes the application of a variety of substances through aqueous or organic solvent base treatments of magnesium-aluminosilicate fibers to enhance temperature resistance of the fibers. U.S. Pat. No. 4,379,111 describes uniformly coating fibers with chromium oxide to enhance heat resistance over uncoated fibers or non-uniformly coated fibers of the same composition. U.S. Pat. No. 4,492,722 describes the deposition of a surface coating consisting essentially of TiO.sub.2 from an organic solution to extend the upper use temperature limit to approximately 2000.degree. F. These post treatments, however, are expensive processes.
Traditional refractory ceramic fiber (RCF) has an upper temperature use limit of 2300.degree. F. and is inexpensively produced in several forms, but all of these forms are discontinuous. Continuous filament ceramic fibers are known, but they must be produced through expensive sol-gel routes.
There are many advantages of fiber in continuous form over traditional discontinuous refractory ceramic fiber, including the elimination of shot, the ability to chop filaments to any desired length, the ability to utilize the filaments directly for the manufacture of textile products, product performance characteristics, and the ability to custom manufacture fibers of known diameter.
Shot refers to high density, unfiberized particles which are detrimental to thermal efficiency, act as stress risers in reinforcing applications, and cause excessive wear in friction applications.
The ability to chop continuous filaments to any desired length is particularly advantageous in papermaking and in reinforcing applications.
The ability to utilize continuous filaments directly for spinning and weaving for the manufacture of textile products eliminates the cost associated with the organic carriers required with traditional refractory ceramic fibers. Textile products produced with traditional discontinuous refractory ceramic fibers typically have lower tensile strength when compared to products produced with continuous fibers. The continuous filaments can be woven into cloth for fire protection or other high temperature applications.
Since continuous fiber can be manufactured more reproducibly than discontinuous fiber, the custom manufacture of fibers of known diameter, within narrow tolerances, is more facile using fibers in continuous form. In addition, the ability to control fiber diameter, especially the control of fiber diameters to values greater than those considered respirable, is beneficial from the aspect of physiological considerations as well as for thermal and reinforcement performance.